|photo credit: http://thefunnyplace.org/tag/halloween-comic/|
Halloween is not really celebrated in Japan so most students don't have any idea what Halloween is. Hence, some teachers ask ALT's to do a special Halloween lesson. And even if they don't ask, it would still be fun to do something related to Halloween.
Most schools have a lot of restrictions though. No activities beyond the classroom. No treats allowed. No costumes. No scary stuff. Teachers just expect a simple but fun Halloween lesson.